|MadSci Network: Evolution|
Well , that is a good and tough question. Evolution is a very tricky thing to speculate about. For instance, why did we develop 5 fingers instead of 4 or 6, or what was the appendix originally for? The answers to such things aren't very clear-cut, at least not yet. But that doesn't mean we can't at least develop a few ideas about what might have happened. So let's take a look at the brow ridge and first think of some ideas of what it's biological or physical purpose is. In the case of a protruding brow ridge, it might be used for protection of the eyes. If the eyes are set far back in the skull, there is a less likely chance that something can reach the soft tissue of the eye and cause it damage. It might have even helped shield the eye from the glare of the sun. It may also be that large eyes are better for seeing prey or anything for that matter, and that larger eye size may have started to evolve before a larger brain. That would at least account for a protruding brow ridge in early humans. Another characteristic of early humans is that the brain case sloped back more away from the eyes. It you look at pictures of chimpanzees and gorillas, you can still see this. With that in mind, we now have to wonder, why don't modern humans also have protruding brow ridges to the extent of our ancestors? Well, it seems that having eyes set further back could still be a good thing, so that would build a case for still having larger brow ridges. Also, the sun is still something we shield our eyes from. But we do have larger brains, which may have caught up with our large eyes. Also, increasing cranial capacity changed the skull to a more efficient shape for a larger brain. So that may be an answer. Or it could be something due to mate selection. Maybe early humans preferred others with less pronounced brows. Unfortunately there is no real way to test for these theories because the results of evolution take so long to turn out. I would guess it may have been a combination of the latter three ideas. If you are more interested I would do a search on "anthropology of early humans", or "evolution of cranial capacity" and see what you get. Of course, a better alternative is to pay a visit to a local anthropology museum. The exhibits and people there would help you a lot. Good luck.
Try the links in the MadSci Library for more information on Evolution.